public class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        // 递归一定是有退出条件的
        if (root == null) return null;
        TreeNode newRoot = new TreeNode(root.val);
        newRoot.left = mirrorTree(root.right);
        newRoot.right = mirrorTree(root.left);
        return newRoot;
    }
}
